where B and SNR represent the signal bandwidth and signal-to-noise ratio, respectively. From Eq. (REF ) it appears evident that an increase in the signal bandwidth, at a constant SNR, will results in greater capacity. However, the nonlinear nature of SMF makes the problem of maintaining the SNR over extended bandwidths almost impossible. Several models have been proposed throughout the years to describe the nonlinear behaviour of optical fibres including, but not limited to [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}. The Gaussian noise (GN) model [4]}, and variations thereof, have enjoyed widespread popularity due to their reduced complexity and accuracy. The main assumption of the GN model is that nonlinear distortions can be modelled as AWGN process that results in a so-called nonlinear interference (NLI) noise power, which can be conveniently used in combination with Eq. (1) to estimate the capacity of a fibre channel. Recently, due to the renewed interest in broadband transmission systems, the GN model has been extended to included impairments due to ISRS [15]}, [16]}, [17]}, [18]}, [19]}, [20]}.
